In recent years, there are increasing demands for capacity and speed enhancement of radio communication, and researches on methods for improving effective utilization of limited frequency resources have been actively conducted. As one of the methods, a technique utilizing spatial areas is attracting attention. One such representative method is a technique whereby spatial orthogonality in propagation paths is utilized and different data sequences are transmitted using physical channels of the same code at the same time instant and at the same frequency. Examples of such a transmission technique include Space Division Multiple Access (SDMA) (e.g., see Non-Patent Document 1) whereby different data sequences are transmitted to different mobile stations, and a Space Multiplexing (SM) (e.g., see Non-Patent Document 2) whereby different data are transmitted to the same mobile station.
In the above described SM, an apparatus on the transmitting side transmits different data sequences from a plurality of antennas provided on the apparatus on the transmitting side at the same time instant, at the same frequency and using physical channels of the same code for each antenna, while an apparatus on the receiving side separates signals received through a plurality of antennas provided on the apparatus on the receiving side into different data sequences based on a channel matrix indicating a propagation path characteristic between the transmission/reception antennas (hereinafter referred to as “BLAST type”) and thereby enables the efficiency of frequency utilization to be improved. When SM transmission is carried out, it is possible to expand the communication capacity in proportion to the number of antennas if the apparatus on the transmitting side and the apparatus on the receiving side are provided with the same number of antennas in an environment in which there are many scatterers between the apparatuses on the transmitting and receiving sides under a sufficient S/N (signal power to noise power ratio).
Furthermore, in realizing considerable increases in capacity and speed enhancement of radio communication, it is important to improve tolerance to multipath or fading. A multicarrier transmission scheme is one approach to realize this and in particular an orthogonal frequency division multiplexing (OFDM) transmission scheme is adopted for terrestrial digital broadcasting and wideband radio access systems.
One example of a transmission scheme in which SM transmission is applied to this OFDM transmission is described in Non-Patent Document 3. Under this transmission scheme, when there is no multipath that exceeds the length of a guard interval, each subcarrier can be regarded as narrow band transmission, that is, flat fading transmission. For this reason, may examples have been reported where a channel matrix is calculated for each subcarrier and SM transmission is carried out based on the calculated channel matrix H.
Non-patent Document 1: “A Study on a Channel Allocation scheme with an Adaptive Array in SDMA”, Ohgane, T., et al., IEEE 47th VTC, pp. 725-729, vol. 2, 1997
Non-patent Document 2: “Layered Space-Time Architecture for Wireless Communication in a fading environment when using multi-element antennas”, Foschini, G. J., Bell Labs Tech. J, pp. 41-59, Autumn 1996
Non-Patent Document 3: “On the Capacity of OFDM-based Spatial Multiplexing Systems”, IEEE Trans. Communications, vol. 50, pp. 225-234, 2002